Filamentary cathode



y 1953 H. JACOBS ET AL 2,639,996

FILAMENTARY CATI-IODE Filed March 17. 1949 INVENTORS Harold [Jacob-s George Hees BYMXAMZM Patented May 26, 1953 UNITED STATES OFFICE YFILAMENTARY CATHODE Harold Jacobs, "St. Albans, and "(Elma-gr: H'ec's, Woodside, N. Y., assignol s' 'to Sylvan-ta anemic Products Ina, a corporation of Massachusetts Application March .17, 1949, ScrialfNm 81,854

5 Claims. (Cl. 117- 5336 invention relates to oxide coated cathodes suitable for use as thermionic emitters. More particularly it relates to a method of increasing the .rate of activation of oxide coated cathodes.

It has been well recognized that there is a distinct need for the discovery and development of new alloy materials which will provide both copious thermionic emission as well as have desirable hot tensile strength characteristics. This need has manifested itself in that the filamentary h:

cathodes of the prior art had a tendency to break or cause short circuits due to their poor strength qualities while operating in a heated condition.

Furthermore since most of 'these'filamentary type tubes are operated by batteries it is well recognized that it would be of advantage to develop filaments which would give high performance with a minimum of required wattage. It has also been recognized that it would be helpful to obtain a filament which would activate quickly to a condition of high stable emission capabilities. From a production standpoint this would save costly time in activation processing on exhaust and on aging.

Previous investigators for alloys for this use have shown that impurities in nickel core material can enhance the thermionic emission of these filaments. The theory for this being based on the fact that the core metal was the source of the emitted electrode. While the theory has been questioned it has been shown without doubt that metal impurities have an appreciable effect on thermionic emission. Other investigators Working with the same nickel. cathodes have indicated that impurities in nickel could enhance or lower thermionic emission. The theory advanced at that time was that the impurities in the nickel diffused to the oxide and reduced the coating providing free barium which in turn lowered the work function.

Although new materials such as cobalt aluinum alloys and nickel aluminum alloys now have been developed for use as base metals for making cathodes having copious thermionic emission and desirable hot tensile "strength 'characteris'ti'cs, these materials have been found to possess the undesirable property of requiring as much as one hundred hours of activation before a high emission value can be obtained in the cathode structure. Naturally such lengthy activation periods are very undesirable from an economical production viewpoint.

The object of this invention is to provide a method of increasing the rate of activation of oxide coated cathodes.

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A still further defect of this invention is to provide a method of manufacuning oxide coated eathode'skhaving a'filamentary core of cobalt aluminum or nickel aluminum which can be activatedmore readily than heretofore deemed possible.

A still further :obj'ect of this invention is to provide an oxide coated cathode having a nickel or cobalt aluminum filamentary "core which can be readily activated.

In accordance with thisfi'nvention, it has been found that these and other objects and advantages can be obtainedb'y introducing nickel'oxide intothe oxide coating materials.

'In the drawings,

Fig. 1 illustrates a .plan view of aspecial emission diode which is adaptable to the study of thermionic electron emission.

Fig. 2 is a trout elevation of the tube shown in Fig.1.

The special electron tube shown in the drawings contains three nickel cylinders 10, 12 and :4 which are used as anodes in a standard soft glass lock in" ty e vacuum tube :bulb I6. The

lament 2D is vertically 'suppor ted'within the tube iii by means of supports 22 and 24. The end cylinders Ill and M are provided with slots [8 which help these cylinders act as guard'rings and help provide a uniform electrost'atic field at that portion of the filament as it is being studied. The use of guard rings also minimizes the end effect, that-is, the cooling of the ends or the filament by thermal conduction. The center anode i2 is isolated electrically and used as the collector for the emission studies. The tube is further equipped with a getter which may be barium aluminum so shielded that it only flashes on the tube walls to abscrbany .gas which may be liberated during life. To reduce the probability of marked evolution of gas no mica or lava spaces are used in the illustrated tube but all spacing is accomplished by means of nickel supports.

The filament -20 is preferablymade of a nickel aluminum alley or of a-cobalt aluminum alloy. Both of these alloys have been found suitable for "the production of cathodes capable of copious thermionic emission and both can withstand high temperatures In the case of the nickel alloy the preferred percentage has been found to 'be approximately '2 atomic percent of aluminum. In the case of the cobalt alloy. the atomic percentage is approximately one percent. A greater percentage of aluminum results in greater emission. In the manufacture of these filaments, it has been found that it is of definite advantage to vacuum melt these alloys before drawing them into wire. This undoubtedly improves the purity of the product. At any rate it certainly has been found to improve its emission characteristics.

The filament 20 is coated with an oxide coatmg capable of giving thermionic emission. For the purpose a triple carbonate mixture containmg a small amount of nickel oxide has been found to be preferable if the best results are to be obtained. Of these substances, the triple carbonate coating is well known and widely used in the art. On the other hand, the addition of the nickel oxide is novel. When this latter ingredient is added, it promotes the activation of the thermionic emitter and materially cuts down the time necessary to reach peak emission. This filament coating can naturally be applied in many ways including spraying and electrophoresis. In accordance with preferred procedure, nickel oxide and the standard triple carbonate suspen sion containing approximately 38.6 mol percent of barium carbonate, 15.2 mol percent calcium carbonate and 46.2 mol percent of strontium carbonate suspended in lacquer is cataphoretically applied to the filament to an approximate oxide diameter of 0.0065 inch, When the filament originally has a diameter of 0.005 inch, the coating density is preferably held within 1.0 plus or minus 0.3 gram per centimeters squared. The amount of nickel oxide used is preferably kept at about 5% by weight of solid material although there is nothing critical in the percentage of this material used. It is preferably added to the triple carbonate in the form of a powder before it is applied to the filament to form the cathode coating.

Although it has been found that the addition of the nickel oxide increases the rate of activation of both the nickel aluminum triple carbonate coated filament and the cobalt aluminum triple carbonate coated filaments, it has been noted that the rate of activation is more marked in the case of the cobalt aluminum alloy base than it is in the case where the nickel aluminum alloy is used as the filamentary material. This however, is not too unexpected in view of the fact that the triple carbonate coated cobalt aluminum alloy without the addition of the nickel oxide to the oxide coating takes much longer to activate than does the nickel aluminum based filament. It is therefore readily apparent that in view of the fact that the cobalt aluminum alloys are the slowest to activate, the nickel oxide is especially useful in the coating of these materials since they present the greatest problem from the point of view of activation time.

Although the nickelous oxide when added to the triple carbonate to form the coating does show an appreciable decrease in activating time for the coated filament, the nickelous oxide does not seem to have any additional efiect that is to say, when once the filament material has reached its highest level of thermionic emission, it is substantially the same as that for filaments coated with triple carbonate but Without the addition of the nickelous oxide, On the other hand, in this connection, it should also be noted that the nickelous oxide has no harmful effect when once the filament has been properly activated.

In actual tests it has been found that the addition of as little as 2% nickel oxide by weight to the triple carbonate coating will decrease the activation time of the nickel aluminum alloy to such an extent that the thermionic emission can readily be obtained within the first twenty-four hours. On the other hand, when the more slowly activated cobalt aluminum alloys are used as the base for the filamentary materials, substantially peak thermionic emission can be obtained within twenty-four hours as compared to one hundred hours when no nickel oxide is used in the coating.

It should, of course, be understood that although the specification has emphasized the use of this coating material on filamentary cathodes, the coating has a general application and can be used beneficially on other types of cathodes.

While the above description and the drawings submitted herewith disclose preferred and practical embodiments of the filamentary cathode of this invention, it will be understood by those skilled in the art that the specific details as shown and described are by way of illustration and are not to be construed as limiting the scope of the invention.

What is claimed is:

l. A filamentary cathode capable of giving copious thermionic emission comprising a core of an alloy selected from the group consisting of nickel aluminum containing approximately 2% aluminum and cobalt aluminum containing approximately 1 aluminum with an electron emissive oxide coating containing 2 to 5% nickel oxide.

2. A filamentary cathode capable of giving copious thermionic emission comprising a core 01' a nickel aluminum alloy with an electron emissive oxide coating containing 2 to 5% of nickel oxide, said alloy containing substantially 2% of aluminum.

3. A filamentary cathode capable of giving copious thermionic emission comprising a core of a cobalt aluminum alloy with an electron emissive oxide coating containing 2 to 5% of nickel oxide, said alloy containing approximately 1% of aluminum.

4 A filamentary cathode capable of giving a copious thermionic emission comprising a core of a nickel aluminum alloy with an electron emissive oxide coating containing approximately 5% of nickel oxide said alloy containing substantially 2% of aluminum.

5. A filamentary cathode capable of giving copious thermionic emission comprising a core of a cobalt aluminum alloy with an oxide coating containing a mixture of barium, strontium and calcium oxides together with 2-5% of nickel oxide said alloy containing approximately 1% of aluminum.

HAROLD JACOBS. GEORGE I-IEES.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,925,978 Cooper Sept. 5, 1933 2,003,609 Riggs June 4, 1935 2,041,802 Wilson et a1 May 26, 1936 2,049,372 Hamada et al July 28, 1936 2,081,864 Edwards et al. May 25, 1937 2,085,605 Ramsey et al June 29, 1937 2,103,267 Mandel et a1. Dec. 28, 1937 2,185,410 Lederer Jan. 2, 1940 

1. A FILAMENTARY CATHODE CAPABLE OF GIVING COPIOUS THERMIONIC EMISSION COMPRISING A CORE OF AN ALLOY SELECTED FROM THE GROUP CONSISTING OF NICKEL ALUMINUM CONTAINING APPROXIMATELY 2% ALUMINUM AND COBALT ALUMINUM CONTAINING APPROXIMATELY 1% ALUMINUM WITH AN ELECTRON EMISSIVE OXIDE COATING CONTAINING 2 TO 5% NICKEL OXIDE. 